Note that Lua 5.1 has switched to the # operator. #t is the replacement for table.getn(t)

> print( #{1,2,3} )
3

However, it does not always behave as one might expect. Consider the following sequence:

> t = { 1, 2, 3 }
> dump (#t)
3

> t[1] = nil
> dump (#t)
3

> dump (t)
{
[2] = 2
[3] = 3
} -- table: 0x807ff70

> dump (#{ [2] = 2, [3] = 3 })
0

As you can see, the # operator is unpredictable in that it can return different values for tables with identical contents. The Lua 5.1 reference manual has this to say about it [1]:

The length of a table t is defined to be any integer index such that t[n] is not nil and t[n+1] is nil.

In other words, be careful with this operator if you are using tables with holes in them. If your table has all its values between [1] and [n] for some n, with no nils in that range, the # operator will return n. Otherwise its result may be unclear.

Note:
The reason the returned values are different is because the tables are different. For indexed tables, or tables that start with index values like 1,2,3 for keys, use tinsert and tremove to change the contents. What # returns is actually correct, setting the index value to nil does not change the size of the table for index keys, tremove would.